This invention relates generally to liquid storage tanks and more particularly to an improved structure for automatically mixing the contents of a liquid storage tank such as a drinking water distribution reservoir.
Drinking water distribution reservoirs, such as standpipes, ground storage tanks, or elevated tanks, provide a reserve of water that can be used to meet short-term periods of high demand. Water is usually pumped into and drawn out of a lower portion of the reservoir. Although the inflow of water creates some turbulence, the turbulence generally is inadequate to provide significant mixing in the reservoir. Consequently, absent mixing, the last water added to the tank would typically be the first water to be removed.
The water near the top of the reservoir, on the other hand, would typically be the last water to be removed, and thus would be removed only in periods of exceptionally high demand. Because it would be the last water to be removed, it could reside in the reservoir for a long period of time. During that time, disinfectant in the water may dissipate and the water could become stagnant, leading to microbial growth and the production of disinfection byproducts. Stagnant water may contain pathogenic, taste, and odor-forming organisms, and may not meet regulatory requirements.
To avoid this problem, distribution reservoirs are often equipped with mixing systems. However, many conventional mixing systems are relatively expensive to build, maintain, and operate. The CB&I Fresh-Mix system described in U.S. Pat. No. 5,735,600, on the other hand, provides a good, relatively-inexpensive mixing system.
In the Fresh-Mix system, a draft tube is positioned above the inlet to the tank. As water flows into the tank, it enters the lower end of the draft tube, pulling other water from the lower section of the tank with it. The water mixes and exits through the upper end of the draft tube. This movement of water through the draft tube develops a rotational flow pattern in the tank, providing an automatic, relatively-inexpensive, and easily-maintained mixing system.
However, there are circumstances when a simple draft tube arrangement may not provide optimal mixing. When the density of the water entering the tank is significantly different than the density of the water already in the tank, a traditional draft tube arrangement may not provide optimal mixing. If the density of the incoming water is significantly greater than the density of the water already in the tank, inflowing water may not reach the top of the draft tube, preventing the desired rotational flow pattern from developing. If the density of the incoming water is significantly less than the density of the water already in the tank, the inflow may tend to accumulate at the top of the tank, creating stratification and again impairing the development of the desired flow pattern.
The efficiency of a draft tube system can also be impaired by a reduction in the liquid level in the reservoir. When the liquid level in the reservoir falls below the top of the draft tube, the mixing pathway through the draft tube effectively shuts down and the mixing ends.
Using a relatively short draft tube might reduce the frequency of the liquid level falling below the top of the draft tube, and thus might reduce the frequency of this problem. However, reducing the length of the draft tube also reduces the mixing provided by the draft tube.
It is therefore desirable to provide an alternative mixing arrangement that addresses one or more of these special problems associated with drinking-water reservoirs.